In the past there have been many attempts to provide tire pressure monitoring in vehicles. Numerous patents have issued which disclose various approaches. Earlier designs tended to be of an electromechanical nature, with large inductive coil mechanisms attached to the wheel and brake drum. These systems provided a high level of information, as to both tire pressure and tire temperature, on a continuous basis. However, high cost and installation difficulties made such systems unacceptable to the majority of mainstream car companies.
More recently, lower cost systems have been developed which provide a simple threshold pressure warning by means of a pressure switch, an internal power source, and a radio wave communication link from each wheel to a dashboard receiver and display. These systems are more attractive to car companies because of their low cost and ease of installation. However, these systems have a number of problems, including, inter alia, the ability to provide a long-lasting and reliable power source, the size and weight of the wheel unit, and mounting of the unit inside the wheel. Moreover, the information available in such systems has been limited to a single pressure threshold, typically 25 psi, which may or may not be compensated for changes in temperature which occur inside the tire. Also, such systems, some of which are not mounted internally of the tire, have been unable to compensate for changes in altitude, which can affect the performance of tire pressure monitoring systems.